Many design ways are used in designs of screws, such as single section, double sections, single thread, double threads, single diameter, double diameters, single pitch angle, double pitch angles, etc. All these designs present different effects in use of screws, such as the effects of drilling speeds, dregs removing, and retaining.
In the design of the screws, the designs of screws are based on the characters of the material and the object to be drilled. Generally, screws are made of different material, such as woods, plastics, metals, compound materials, etc. For a screw screwing into a wood, due to the property of the wood, it is often that the wood will crack. Thus it is necessary to have screws which have the property of crack proof.